1. Field of the Invention
This invention relates to the treatment of used linings of aluminum reduction cells, commonly referred to as spent pot linings. More particularly, the invention relates to the treatment of linings of this type to permit safe disposal of the linings.
2. Description of the Prior Art
Aluminum is normally produced by dissolving alumina at high temperature (about 900xc2x0 C.) in molten cryolite (Na3AlF6) in electrolytic cells, or pots, provided with electrically conductive carbon linings, and electrolyzing the molten solution by passing an electric current between carbon anodes dipping into the melt and the carbon linings acting as cathodes. Cells of this type may be used for considerable periods of time, e.g. up to ten years, and during this time the carbon lining material absorbs sodium fluoride and other contaminants. At the end of the operational lifetime of the cells, the linings are removed and broken up and have to be disposed of in some way. However, the spent lining material, which is composed of carbon, refractory material from insulating refractory bricks and cryolite, including fluorine, aluminum, sodium, calcium and silicon values, along with free and complexed cyanides, carbides and nitrides, is hazardous and must be treated with great caution.
The electrolytic cells for producing aluminum are typically of two types. The first is a pre-bake type, where the carbon-based anodes are first formed and then are baked under high temperature to maintain their shape without support in the cell. The second is the Soderberg cell where the anode material is semi-fluid and requires an open-ended box-like container to hold it in place.
The safe disposal of spent linings has for a long time presented a challenge to the industry. That challenge continues with ever stricter environmental standards. Thus, disposal residues are limited to very low concentrations of fluorides and cyanides, e.g. TCLP (Toxicity Characteristic Leaching Procedure) leachable fluorides of less than 49 mg/l and reactive cyanides of less than 250 mg/kg as HCN.
Previous researchers have come up with a variety of possible methods for treating spent pot linings. For example, Bell et al. in U.S. Pat. No. 4,113,832 describe a process for treating spent pot lining in which the crushed material is first subjected to a high temperature pyrohydrolysis treatment in the presence of water at 1,150 to 1,250xc2x0 C. The NaF and HF off-gases in vapour form are obtained and are recovered. The solid residue is immersed in a dilute caustic solution at greater than 200xc2x0 C. to leach out the alumina for later reuse. This process requires very large and expensive reactors and their high capital and operating costs make them uneconomical to operate.
In Lever, U.S. Pat. No. 4,816,122 a process for treating spent pot lining material is described in which the spent material is treated with caustic solutions of either high or low concentration. In the high concentration option, the waste material is first leached in a 200-400 g/l caustic solution at less than 100xc2x0 C., followed by leaching in water at less than 100xc2x0 C. It has been found that this process traps too much fluoride in the residue such that it is not capable of meeting government environmental requirements.
Snodgrass et al. U.S. Pat. No. 4,444,740 describes a process for treating spent pot linings in which crushed pot lining material is first incinerated to destroy the cyanides. The resulting ash is then leached with water or dilute caustic at 20-120xc2x0 C. to recover the fluoride values. Incineration is an expensive procedure for this purpose.
Another patent which uses calcination to destroy the cyanides is described in Lam et al U.S. Pat. No. 3,808,322. In that case, the calcined material is subjected to a water leaching step at 50-100xc2x0 to recover the fluorine values.
A more recent patent also relating to the treatment of spent pot linings is Grolman et al. U.S. Pat. No. 5,470,559. In that process, the spent pot lining material is first treated with an aqueous sodium hydroxide solution. Thereafter, the solution obtained is heat treated to destroy cyanide values and water is evaporated in the resulting solution to cause fluoride compounds in the solution to precipitate. The fluoride crystals are then separated from the solution.
It is an object of the present invention to provide an improved leaching process for removing the environmentally harmful materials, such as fluorides and cyanides, from the spent pot linings and render the residue safe for landfill as a non-hazardous bi-product, or further recuperation of chemical values.
The present invention provides a process for treating spent pot lining material contaminated with fluoride and cyanide values. The spent pot lining material is first leached with water at a dilution and time sufficient to dissolve substantially all water soluble compounds including fluorides in the spent material. The solution is then removed from the solid residue and this residue is then subjected to a caustic leach with an aqueous sodium hydroxide solution containing about 20 to 50 g/L of sodium hydroxide. Thereafter, the solid residue is separated from the liquid.
The first stage water leaching is preferably carried out at a dilution ratio of spent material:water in the range of 1:3 to 1:8, more preferably 1:3 to 1:4. The water leaching is typically carried out at a temperature in the range of about 20 to 70xc2x0 C. for a period of about 10 to 20 minutes.
The second stage caustic leaching is preferably carried out at a dilution ratio of spent material:water in the range of about 1:4 to 1:12, with the ratio of 1:6 being particularly preferred. Caustic concentrations in the range of about 30 to 40 g/L are also particularly preferred.
It is quite surprising that the caustic leaching process is greatly improved by preceding this with an initial water leaching stage. By initially removing all soluble fluorides, the level of caustic in the second stage caustic leaching need only be sufficient to attack and break down the cryolite (sodium aluminum fluoride compounds) so that the remaining soluble fluoride is released, but at a level not so high as to depress the solubility of that fluoride. It has been found that with the process of this invention, the total combined water/caustic leachate volume is 20% smaller than if just caustic by itself was used for leaching. This results in the size of the cyanide reactor and evaporator/crystallizer units being reduced and translates into lower cost for the process as a whole.
It has been found that for certain types of pre-bake electrolysis cells, an additional water leaching step may be necessary following the caustic leaching to remove all of the fluoride. For the Soderberg cells, in which the residue contains lithium, both an acid activation step and a further caustic leaching step may be necessary to overcome the protective effect of lithium on the soluble fluoride. When an acid activation step is used, it is typically at a pH of about 7 to 10, preferably 8 to 9.